The present invention relates to a method and device for optically measuring the gloss or reflectance of a surface, and more particularly, to a gloss sensor having increased sensitivity to changes in the gloss of a surface and to a method for increasing the sensitivity of gloss sensors.
One of the parameters used in determining the quality of a particular surface is the gloss of the surface. For example, in the papermaking industry, various grades of paper having different surface gloss qualities are produced to suit various applications. During the production of paper sheet, it is often desirable to continuously or periodically measure the gloss of the surface of the sheet to ensure that the surface of the sheet being produced has the desired gloss. Typically, the gloss of paper is measured with a gloss sensor during the last step of paper production just before the produced sheet is packaged in the form of rolls.
Known devices for determining the gloss of surfaces include, for example, an optical system which measures the intensity of a beam of light reflected from the surface under test. Typically, the gloss of such a surface is determined by comparing its total reflectance (specular and diffuse) to the reflectance of a gloss standard, such as, for example, a glass tile having a polished surface of a known gloss. "Specular reflectance" refers to reflections of the type produced from a polished, perfectly flat surface or mirror. "Diffuse reflectance", on the other hand, refers to scattered reflections produced by irregular surfaces. Because no actual surface is perfectly specularly reflective, reflections from any real surface will have both a specular component and a diffuse component.
Specifically, in measuring the reflectance of a surface, an incident beam of light is first projected onto the surface from a light source and a resulting reflected beam, having both specular and diffuse reflection components, is reflected from the surface. A light intensity detector is positioned to receive the reflected beam and measure its intensity. To determine the gloss of the surface, the intensity of the reflected beam is compared with a known intensity value, such as the intensity of a reflected beam which results when the incident beam from the light source is reflected from a polished glass tile having a known gloss. This comparison can be accomplished with known electronic devices.
Typically, a gloss sensor viewing angle is fixed. That is, the angle which the incident and reflected beams make with the normal to the surface plane remains constant during the reflectance measurement of that surface. Different angles, with respect to the normal to the surface plane, are used for different sheet reflectivity measurements, with smaller angles being employed for more reflective surfaces. For reflectance measurements of paper, the standard angles, with respect to the normal to the plane of the paper, are 20.degree. for paper having a high reflectivity surface and 75.degree. for paper with a low reflectivity surface.